1. Field of the Invention
The present invention relates to ink droplet ejection apparatus for ejecting ink to form an image on a recording medium.
2. Description of Related Art
Conventional recorders which are easy of multiple gradation and colorization include ink jet recorders. Of these recorders, ink droplet ejection apparatus of the drop-on-demand type, which eject printing ink, are coming into wide use because of high ejection efficiency and low running costs.
For example, Japanese Patent Application Laid-Open No. 63-247051 discloses ink droplet ejection apparatus of the shear mode type as ink droplet ejection apparatus of the drop-on-demand type. Piezoelectric material is used in the apparatus disclosed in the publication. FIGS. 23 and 24 of the drawings accompanying this specification show part of a conventional ink droplet ejection apparatus of the shear mode type.
The apparatus shown in FIGS. 23 and 24 includes a 64-channel multi-nozzle print head 600, only five channels of which are shown for simplification. The head 600 includes a bottom wall 601, at top wall 602 and a number of shear mode actuator walls 603 extending between them. Each actuator wall 603 includes an upper part 605 and a lower part 607 which are made of piezoelectric material. The wall parts 605 and 607 are bonded to the walls 602 and 601, respectively, and polarized oppositely as shown by arrows 609 and 611, respectively. The actuator walls 603 are arranged in pairs to define channels 613 between them. Spaces 615 are defined between successive pairs of actuator walls 603.
At one end of the channels 613 is secured a nozzle plate 617 formed with nozzles 618 for the respective channels. The other end of the channels 613 is connected through a manifold 626 to an ink cartridge or another ink supply (not shown). The manifold 626 includes a front wall 627 and a rear wall 628. The front wall 627 is formed with holes communicating with the respective channels 613. The rear wall 628 closes the space in the rear of the front wall 627 between the rear ends of the base wall 601 and top wall 602. Ink can be supplied from the supply to the space between the front wall 627 and rear wall 628, and then be distributed to the channels 613.
Electrodes 619 and 621 are disposed on opposite sides of each actuator wall 603. The electrodes 619 disposed in the channels 613 are connected to a drive circuit 21. Under the control of a control circuit 22, the drive circuit 21 can generate or output a voltage and apply it to these electrodes. The electrodes 621 disposed in the spaces 615 and on both sides of the print head 600 are connected to a ground return 623.
In operation, the voltage applied to the electrodes 619 in each channel 613 causes the actuator walls 603 facing the channel to deform piezoelectrically in such directions that the channel enlarges in volume. For example, if, as shown in FIG. 25 of the accompanying drawings, a voltage of E volts is applied to the electrodes 619 in one of the channels 613, electric fields are generated in the actuator walls 603 defining this channel. As shown by arrows 631 and 632, the fields are normal to the directions 609 and 611 of polarization. This deforms these walls 603 piezoelectrically in such directions that the channel 613 enlarges to reduce the pressure in the channel to a negative pressure.
The voltage applied to the electrodes 619 is held for a period L/V where L is the channel length and V is the sound velocity in the ink in the channel 613. While the voltage is applied, ink is supplied from the supply to the channel 613. The period L/V is the one-way propagation delay time T which it takes for the pressure wave in the channel 613 to be propagated one way longitudinally of the channel.
According to the theory of pressure wave propagation, the negative pressure in the channel 613 reverses into a positive pressure when the period L/V passes after the voltage is applied to the electrodes 619. If the voltage is returned to zero volt when this period passes after the voltage application, the deformed actuator walls 603 return to their original condition shown in FIGS. 23 and 24. This applies a positive pressure to the ink in the channel 613. This pressure is added to the pressure reversed to be positive. As a result, a relatively high pressure develops in that portion of the channel 613 which is near to the associated nozzle 618, ejecting ink out through the nozzle. The ejected ink sticks to a surface of printing paper or another recording medium to form an image on it.
The present assignee's Japanese Patent Application Laid-Open Nos. 9-29960, 9-29961 and 9-48112 disclose the step of ejecting ink out through a nozzle 618 by generating pressure wave vibration in the ink in the associated channel 613, and the step of substantially canceling the residual pressure wave vibration of the ink in the channel after the ejection. This cancellation involves generating an additional pulse after the main drive waveform for the ejection. Specifically, the cancellation involves increasing and decreasing the volume of the channel 613 by applying a voltage of E volts to the associated electrodes 619 at a predetermined time after the ejection and by subsequently returning the voltage to 0 volt. The cancellation damps the residual pressure wave vibration in the channel 613 quickly and early. This prevents ink from being ejected or dropped accidentally through the nozzle 618 by the residual pressure wave vibration. Besides, this enables early transition to the process in accordance with the next print command for this channel. It is therefore possible to form a more exact image on a recording medium, and improve the print speed.
The assignee's Japanese Patent Application Laid-Open No. 10-202858 discloses the steps of ejecting ink out through a nozzle 618 in a print cycle, and thereafter canceling the residual pressure wave vibration in the associated channel 613 if there is no print command for this channel for the following cycle, ibut canceling no such vibration if there is a print command therefor. If there is no print command for the following cycle, ;an accidental drop of ink may occur, and therefore the residual pressure wave vibration should be canceled. This results in better image formation not stained or spotted by scattered ink. If there is a print command for the following cycle, the residual pressure wave vibration in the channel 613 should be utilized positively. Specifically, this vibration should be added to the pressure wave vibration generated in accordance with the print command for this cycle. The addition generates greater pressure wave vibration for ejection of a larger ink droplet through the nozzle 618. Larger ink droplets increase the print density to form a thicker and clearer image.
It is conceivable that it is possible to damp or control ink meniscus vibration more effectively by switching between the execution and no execution of the vibration cancellation for a particular channel in a particular print cycle selectively depending on, not only whether there is a print command for the following cycle for this channel, but also whether there is a print command for the preceding cycle for the channel. This would stabilize the droplet jet velocity and the ejection, and make it possible to obtain an ink droplet of desired volume, thereby improving the print quality.
The viscosity and other characteristics of the ink in an apparatus vary with the temperature and other conditions at or in which the apparatus is used. It is desirable to perform the foregoing switching arbitrarily and easily depending on this variation as well. It has been desirable to generate a stop pulse for the vibration cancellation securely by means of simple structure or construction, depending on the preceding and/or following print data. It has also been desirable to ease the restrictions on the print waveforms for the cancellation.